One of the main protagonists, and a powerful Vanguard player who attends Hitsue High School. Kai only likes to battle strong players, and dislikes those who run away or cannot back up their boasts with skill. He battles with a Kagero deck with his ace cards being Dragonic Overlord and later Dragonic Overlord the End. He has a few trademark quirks when fighting, such as adding "the" to his move declarations (e.g. - "Stand up! THE vanguard" and "Check THE drive trigger!") and declaring "Final Turn" when he is confident he will win by the end of the current turn.
Although he has a cold personality, Kai is shown to be concerned with Aichi's development, especially after Aichi awakens the power of Psyqualia. Having previously witnessed the corrosive effects of Psyqualia on his former friend Ren, Kai attempts to dissuade Aichi from using Psyqualia by telling him he lacks true strength and then quitting the team. However, his words have the opposite effect intended, making Aichi grow more reliant on Psyqualia for strength. It is later revealed that Kai's true purpose for leaving the team was to train alone and grow strong enough to defeat Aichi, saving him from Psyqualia's influence.
In the second season, his Kagero deck was also switched to Narukami as his first deck was sealed away; his new key card is Dragonic Kaiser Vermillion. Kai has also joined New Team Asteroid. He later gains Dragonic Kaiser Vermillion "THE BLOOD".
In the third season, Kai is one member of his school's Vanguard Club along with Miwa, Morikawa, and Izaki. Kai changes his Narukami deck to the Eradicator archetype, and his new